A sound wave is a kind of wave, and therefore the sound wave is refracted to a different direction of propagation while passing through a different medium. Recently, in the field of ultrasonic wave, an active research has been in progress in focusing sound by forming a micro focal point at a specific distance from the ultrasonic source. However, it is not technically easy to focus sound in an audible frequency band due to diffusing characteristics of sound wave in a wave projecting direction. Though a conventional sound focus system including a sound reflector can be implemented rather theoretically, the component of the system is heavy weight and a rich sound quality cannot be guaranteed.
Further, due to the wave characteristic of the sound wave, the sound waves interfere with one another. Therefore, at a practical circumstance, a sound level heard by a listener is substantially attenuated. And the emitted sound wave may cause a background noise because the sound wave is spread to a certain peripheral area.
By using a refraction of sound wave, a speaker for focusing a sound to a focal point and reducing its spreading to an outskirt of the focal point is required.